This invention pertains to glass compositions having low thermal expansion and low dielectric constant.
In one of its more specific aspects, this invention pertains to such glasses which can be fiberized to produce continuous, defect-free fibers.
Recently, there have been developed integrated circuits mounted in leadless ceramic chip carriers. Such circuits comprise a glass fiber-filled laminate, or board, with an electrical chip affixed thereto. It is important that the board has certain properties. One of these is that the board has the same, or nearly the same, coefficient of thermal expansion as the chip to prevent separation therebetween during operation. Also, it is important that the board have a low dielectric constant to allow high electrical signal propagation speeds.
To meet these requirements, the glass fibers which are employed should have a coefficient of thermal expansion (CTE) less than about 2.3 parts per million per Kelvin (ppm/K) and a linear, elastic (Young's) modulus greater than about 10 million pounds per square inch (Mpsi). Furthermore, the viscosity of the molten glass, expressed as the temperature at which the viscosity is 10 to the power 2.5 poise, is less than about 2850.degree. F. and the liquidus temperature should be 300.degree. F. or more below this viscosity. The dielectric constant should be less than about 6 at 1 million Hertz (MHz).
The following glass types are known in the trade and typically have the following properties:
______________________________________ E glass S glass Silica glass ______________________________________ CTE 5.0 2.2 0.6 ppm/K Modulus 10.5 13.0 10.0 Mpsi Viscosity 2400 2890 -- .degree.F. Liquidus temp. 2000 2700 3100 .degree.F. ______________________________________
Each of these glass types fails to meet all of the requirements in some important way. For example, E glass has a low viscosity and liquidus temperature but an unacceptable CTE. Silica (silicon dioxide) glass fibers have a very low thermal expansion coefficient but have a low modulus and such a high viscosity and liquidus temperature that they may not be fiberized from the melt.
Furthermore, the properties of the aforementioned glasses are typical of the commonly known silicate glasses. Low thermal expansion is generally associated with glasses with high silica content such as silica and S glass. But such glasses usually have such very high viscosity or liquidus temperature or both. Thus, it is surprising that all of the aforementioned properties could be simultaneously met in a silicate glass composition.
The glass of this invention meets those requirements.